Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy

doi:10.1016/j.jmst.2017.07.016

Abstract

Abstract:

The partitioning effect of Al (α-phase stabilizer) and V elements (β-phase stabilizer) on strength of the primary α phases in the α/β Ti-6Al-4V alloy with the bimodal microstructure was investigated. It was found that partitioning of Al and V elements took place in the Ti-6Al-4V alloy during the recrystallization process, leading to the variation of the content of Al and V elements in the primary α phases with changing the volume fraction of the primary α phase. Nanoindentation tests reveal a general trend that the strength of the primary α phases increases with decreasing the volume fraction of the primary α phases, and such trend is independent on the loading direction relative to the c-axis of the α phase. The enhanced strength is attributed to the increase of the content of Al element in the primary α phase, but it is not dominated evidently by the change of the V content. The solid solution strengthening contributed from both the elastic strain introduced by the solute atoms and the variation of the density of states was estimated theoretically.

Key words: Ti-6Al-4V, Nanoindentation, Strength, Alloy element partitioning, Crystallographic orientation

L.R. Zeng, H.L. Chen, X. Li, L.M. Lei, G.P. Zhang. Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy[J]. J. Mater. Sci. Technol., 2018, 34(5): 782-787.

Figures/Tables 9

Fig. 1. Schematic diagram of heat treatment routes for obtaining bimodal microstructures in α/β Ti-6Al-4V alloys with different volume fractions of the primary α phases.

Fig. 2. SEM micrographs showing microstructures of the alloys with different volume fractions of the primary α phase of: (a) 36 vol.%, (b) 50 vol.%, (c) 60 vol.%, and (d) 76 vol.%.

Table 1 Solute concentration of Al and V elements and mean grain size of αp in different samples.

Sample	36% α_p	50% α_p	60% α_p	76% α_p
dα_p (μm)	7.1 ± 2.5	9.5 ± 3.3	8.2 ± 2.4	7.7 ± 2.6
Al (at.%)	11.59	11.31	11.19	10.77
V (at.%)	1.58	2.03	2.03	2.84
Al + V (at.%)	13.17	13.34	13.22	13.61

Fig. 3. (a) SEM quadrant back-scattering detector (QBSD) image of the sample with 60 vol.% αp phase, showing an array of indents numbered by 0-59 made on the sample; (b), (c) and (d) EBSD images corresponding to (a).

Fig. 4. (a) Load indentation depth curves of the [25\(\overline{7}\)3]-oriented αp phases in the samples with 36 vol.% αp and 76 vol.% αp, (b) variations in hardness with the declination angle (γ) of the loading direction relative to the c-axis in the samples with different volume fractions of the αp phases, (c) mean hardness and modulus of samples with different volume fractions of the αp phases.

Fig. 5. (a) STEM-HAADF image of the αp and αs phases. The inset shows the STEM-EDS mapping which confirms alloy element partitioning, (b) line scanning analysis of local chemical composition in the position marked by the red line in (a), (c) contents of Al and V elements in samples with different volume fractions of the αp phases.

Table 2 Calculated lattice parameter via the first principle method using the virtual crystal approximation.

	Pure Ti	α_p = 76%	α_p = 36%
Lattice parameter (nm)	2.9592	3.2332	3.2939

Table 3 Calculated elastic and potential electronic effects on strength of αp phase.

Sample	Strength (MPa)	Contribution from elastic effect (MPa)	Contribution from potential electronic effect (MPa)
Pure Ti	700 [31]
36%α_p	1460	315	445
76%α_p	1220	265	255

Fig. 6. Density of states (DOS) of pure titanium 36% αp and 76% αp estimated by the first principle method using the virtual crystal approximation. Here, the Fermi level is indicated by the vertical dotted line.

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